Distinguishing automatically between genuine U.S. coins and other objects such as foreign coins, counterfeit coins, and metallic slugs is of great importance in devices such as coin operated vending machines and automatic toll booths, as well as in many other, similar devices.
Many methods have been suggested to accomplish the task of distinguishing genuine coins from counterfeit ones. A number of these methods rely on purely mechanical devices which both weigh the coin and measure its physical dimensions. Such devices generally operate relatively slowly, which may be a problem if the device is required to operate in a high-volume, high speed area. Additionally, the capability of such devices to detect well made counterfeits is limited. Finally, a mechanical device, due to size limitations and relative mechanical complexity, can only operate upon and distinguish between counterfeit and genuine coins for a limited number of coin types.
Various electrical methods have also been suggested to distinguish between counterfeit and genuine coins. Many of these involve the use of an inductive coil which forms part of an electromagnetic "tank" circuit. When a metallic coin-shaped object passes through the magnetic field of the inductive coil, the inductance and hence the circuit's resonance changes, depending upon coin size, composition and magnetic permeability. These changes can be detected by "ringing" (periodically applying a voltage to the circuit) the resonant circuit when a coin-like object is present to produce a damped resonant waveform output. A comparison between the resultant decaying waveform in frequency and/or decay characteristics with stored values for genuine coins allows the circuit to distinguish the genuine coins from the counterfeit.
Existing counterfeit coin detector circuits have several problems. The first is that the sensing circuit is generally quite susceptible to variations in temperature. Thus, as temperature changes, genuine coins may be determined to be counterfeit, or counterfeit coins may go undetected. This is a particular problem in coin detectors which must operate in an outdoor environment, such as automatic toll collecting systems for roadways.
Another problem is that most counterfeit coin detector circuits can only validate a limited number of different types of coins. In an area adjacent to a national border, where two different sets of coins may appear relatively frequently, this limitation can be quite troublesome. Finally, the purely analog nature of these circuits requires relatively frequent calibration over the lifetime of the circuit; not merely to correct the effects of temperature changes, but also to compensate for wear on the mechanical portions of the equipment, humidity, etc.
Thus, there is a need for a counterfeit coin detector circuit which can validate many different types of coins, which can operate very rapidly with a very low error rate in a relatively harsh, outdoor environment, that is simple and rugged, which requires very little, if any, calibration and which is highly temperature independent.